In the manufacture of paper from wood, the wood is first reduced to an intermediate stage in which wood fibers are separated from their natural environment and transformed into pulp, a viscous liquid suspension. Several techniques are used to produce pulp from various types of wood. The simplest of these techniques is the refiner mechanical pulping (RMP) method, in which the input wood is simply ground or abraded in water through a mechanical milling operation until the fibers are of a defined desired state of freeness from each other. Other pulping methodologies include thermo-mechanical pulping (TMP), chemical treatment with thermo-mechanical pulping (CTMP), chemi-mechanical pulping (CMP) and the chemical pulping, sulfate (kraft) or sulfite processes for pulping wood. The general concept in all of these processes for creating pulp from wood is to separate the wood fibers to a desired level of freeness from the complex matrix in which they are embedded in the native wood.
Of the various components of wood, cellulose polymers are the most abundant and are the predominate molecule desired for retention in pulp for paper production. The second most abundant polymer in wood, which is the least desirable component in the pulp, is lignin. Lignin is a complex macromolecule of aromatic units with several different types of interunit linkages. In the native wood, lignin physically protects the cellulose polysaccharides in complexes known as lignocellulosics. In chemical pulping processes, lignin is removed. In chemi-mechanical processes, lignin is disrupted to free the cellulose or to make it easier to mechanically free the cellulose.
Biological systems can be utilized to assist wood pulping. A desirable biological system would liberate cellulose fibers from the lignin matrix by taking advantage of the natural abilities of an organism. Research in this area has focused on white-rot fungi, so named because the characteristic appearance of infected wood is a pale color. This color is the result of the depletion of lignin in the wood, the lignin having been degraded or modified by the fungi. Because white-rot fungi appear to preferentially degrade or modify lignin, it is a logical choice for biological treatment to pulp wood. Pulping by this method is referred to as "biopulping."
Several attempts to create biopulping systems using white-rot fungi on a variety of wood fibers have been reported. The most commonly utilized fungus is the white-rot fungus Phanerochaete chrysosporium, also referred to as Sporotrichum pulverulentum. Other fungi which have been previously used in such procedures include fungi of the genera Polyporus and Phlebia. The prior art is generally cognizant of the fact that attempts have been made to use microorganisms, such as white-rot fungi, as part of a process of treating wood in combination with a step of either mechanical or thermo-mechanical pulping of cellulose fiber.
Another example is U.S. Pat. No. 3,962,033, directed to the biopulping of cellulose using white-rot fungi. The fungi used included both naturally occurring wild-type strain cultures and mutant strains produced which lacked cellulase, so as to reduce the amount of cellulose degraded by the organisms. Various types of wood were degraded with the fungi. This wood was then used as input materials for a thermo-chemical or thermo-mechanical pulping procedure. This patent discloses various techniques for making a cellulose pulp by depleting lignin while reducing the cellulose-decomposing action of the enzymes produced by these organisms in order to preserve the cellulose yield. Groups working with the inventor of this patent have several publications regarding use of fungi for biomechanical pulping, e.g. Anders and Erikkson, Svensk Papperstidning, 18:641-2 (1975), Erikkson and Vallander, Svensk Papperstidning, 6:85:33-38 (1982).
U.S. Pat. No. 5,055,159 discloses biopulping with Ceriporiopsis subvermispora. Biomechanical pulping of both hardwood and softwood chips with this white-rot fungus has been demonstrated. During this process at a laboratory scale, fungal pretreatment of both hardwood and softwood species saves substantial amounts of the electrical energy during refining, improve paper strength, and reduce the environmental impact of pulping (Akhtar, et al., "Biomechanical pulping of loblolly pine with different strains of the white-rot fungus Ceriporiopsis subvermispora," Tappi J. 75:105-109, 1992; Akhtar, et al., "Biomechanical pulping of loblolly pine chips with selected white-rot fungi," Holzforschung 47:36-40, 1993; Akhtar, et al., "Biomechanical pulping of aspen wood chips with three strains of Ceriporiopsis subvermispora," Holzforschung 48:199-202, 1994; Kirk et al., "Biopulping: A Glimpse of the Future?", Res. Rep. FPL-RP-523, Madison, Wis. pp. 74, 1993). These results show the technical feasibility of biopulping.
One of the key factors determining the commercial and economic feasibility of biopulping is the cost of the fungal inoculum and the related question of culture time of the organism in the wood. Commercial considerations impose a particular time frame on the amount of time, referred to as the dwell time, that can be dedicated to permitting the biopulping fungus to propagate in the wood. One solution to the problem of obtaining sufficient fungal action prior to pulping is to simply add more fungal inoculum. However, the process soon becomes cost prohibitive, if an excessive amount of fungal biomass is needed. Therefore, the art needs a method to reduce the quantity of fungal inoculum needed for successful biopulping in a time scale suitable for commercial application.